Math.easeInOutQuad = function (t, b, c, d) {
	t /= d / 2
	if (t < 1) {
		return (c / 2) * t * t + b
	}
	t--
	return (-c / 2) * (t * (t - 2) - 1) + b
}

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
const requestAnimFrame = (function () {
	return (
		window.requestAnimationFrame ||
		window.webkitRequestAnimationFrame ||
		window.mozRequestAnimationFrame ||
		function (callback) {
			window.setTimeout(callback, 1000 / 60)
		}
	)
})()

/**
 * Because it's so fucking difficult to detect the scrolling element, just move them all
 * @param {number} amount
 */
function move(amount) {
	document.documentElement.scrollTop = amount
	document.body.parentNode.scrollTop = amount
	document.body.scrollTop = amount
}

function position() {
	return document.documentElement.scrollTop || document.body.parentNode.scrollTop || document.body.scrollTop
}

// 滚动条滚动
/**
 * @param {number} to
 * @param {number} duration
 * @param {Function} callback
 */
export function scrollTo(to, duration, callback) {
	const start = position()
	const change = to - start
	const increment = 20
	let currentTime = 0
	duration = typeof duration === 'undefined' ? 500 : duration
	var animateScroll = function () {
		// increment the time
		currentTime += increment
		// find the value with the quadratic in-out easing function
		const val = Math.easeInOutQuad(currentTime, start, change, duration)
		// move the document.body
		move(val)
		// do the animation unless its over
		if (currentTime < duration) {
			requestAnimFrame(animateScroll)
		} else {
			if (callback && typeof callback === 'function') {
				// the animation is done so lets callback
				callback()
			}
		}
	}
	animateScroll()
}
